It is well known to treat water, for example, waste water containing potentially harmful contaminants, by passing it through contaminant-removing filtration media (also referred to herein as “sorbent,” filter media,” or simply “media”) which has been packed into an elongate axial flow column. The contaminant-removing media forms a porous matrix through which the contaminated water flows. The path of the water is generally linear, along the axis of the filtration column with the downward flow of the water taking place under the force of gravity. As the contaminated water passes through the contaminant-removing media, contaminants in the water are removed. The contaminant-removing media may extract particular contaminants of interest via a number of different mechanisms such as absorption, adsorption, ion exchange, affinity, hydrophilic interactions, hydrophobic interactions, size exclusion, and other mechanisms known to those skilled in the art.
Axial flow columns are generally cylindrical and include an inlet at one end of the column and an outlet at the other. When used for commercial purposes, very large columns are sometimes required, with some commercial axial flow columns being, for example, as high as about six meters with a diameter of about three meters.
A problem can occur when increasing the throughput of an axial flow column. The combination of a high flow rate and a large bed height may result in a high pressure drop across the media. This may result in compression of the media which adversely affects the flow patterns through the column. In some areas, flow may be reduced almost to zero, while in other areas, compression of the media can result in the formation of channels in the media which facilitate the passage of contaminated water and greatly reduce the contaminant removal performance of the axial flow column.
One solution to the problems associated with axial flow columns is provided in U.S. Pat. No. 5,597,489, which discloses a radial flow column for water treatment. A radial flow column includes a fluid chamber which has cylindrical inner and outer screens positioned therein. A contaminant-removing media is packed in a media bed between the inner and outer screens. Contaminated water enters the column, and contacts the outer screen. The contaminated water then moves inward through the filtration media towards the inner screen where the treated water exits into the central lumen of the radial flow column. The filtered water can then be removed from the radial flow column through the central lumen.
FIG. 1 shows a longitudinal section of a radial flow column such as disclosed in U.S. Pat. No. 5,597,489. The outer casing 1 contains an outer mesh screen 2 and an inner mesh screen 3 and a filtration media 4 disposed between the inner and outer mesh screens. When viewed in a horizontal sectional plane, the filtration bed is annular in nature. The inner mesh screen 3 defines the lumen 5 of the device. Water enters the device at an inlet 6 and passes into the annular space 7 surrounding outer mesh screen 2. The water then passes through the outer mesh screen 2, filtration media 4, and inner mesh screen 3 before being taken off via the lumen 5 and exiting the device at the output 8.
In devices such as that illustrated in FIG. 1, any deficiencies in the filtration media, for example, variations in the packing density of the media from one portion of the media bed to another, can lead to the formation of channels in the media bed. These channels may be undesirable because they allow for the passage of contaminants through the filtration bed and directly into the treated water. This can result in contaminants being either discarded into the environment or, if the filtration device is being used for drinking water filtration, unknowingly consumed.
Additionally, the nature of the flow in radial flow columns is considerably more complex than those in simple axial columns and so, accordingly, there is a need in the art for a more rational basis on which to design and construct radial flow columns.